Basic Helicopter Aerodynamics, 3rd Edition

Chapter 5

Rotor Aerodynamics in Forward Flight

The aerodynamic situation in forward flight is complex. Numerical methods have largely taken over the task of evaluation but an analytical treatment, using simplifying assumptions, is valuable for providing a basic understanding of rotor behaviour. Such a treatment is the subject of this present chapter. The mechanisms of the previous chapter affect essentially the details of blade element theory. Before turning to that, however, it is useful to examine briefly what can be made of momentum theory, which as has been said is principally a theory for hover and axial flight; also it may be asked to what situations one is led in considering a more detailed wake analysis under forward flight conditions.

5.1 Momentum Theory

As the rotor begins to attain forward flight speed, the velocity of the air entering the rotor will increase as the forward flight speed combines with the sum of the induced velocity and any axial flight speed. In addition, it will approach the rotor disc at an inclined angle, hence any development of the actuator disc theory to include forward flight must address these issues. The type of streamtube shape seen in forward flight with an actuator disc is shown in Figure 5.1.

Figure 5.1 Streamtube in forward flight

The modelling of the actuator disc theory into forward flight was addressed by Glauert [1] who devised a scheme ...